A much better way of referencing the heaters would be to do away with R11 and create a virtual centre tap using 2 low value resistors (100 - 220 ohm). One end of each resistor is connected to each leg of the heater supply and the other ends are connected together to form the centre tap. This can then be connected either to ground or to an elevated DC voltage which could be the power valve cathode or from a voltage divider off the HT supply (30 - 60v would be typical).
Lots of good info on the subject here
http://www.valvewizard.co.uk/heater.html